FIG. 1 shows a configuration of a conventional receiving set. A signal RA received by an antenna 11 is supplied to a low-noise amplifier 12. The low-noise amplifier 12 amplifies the signal RA and supplies it as a received signal RB to a frequency converter 13. The frequency converter 13 is supplied with a signal RL0 having a local-oscillated frequency from an oscillator 14 and so multiplies the received signal RB and the signal RL0 to down-convert the received signal RB so that an intermediate frequency signal RIF may be generated and supplied to a bandpass filter 15.
The bandpass filter 15 limits a band of the intermediate frequency signal RIF and supplies it to a variable gain amplifier 16. The variable gain amplifier 16 amplifies the signal supplied from the bandpass filter to a predetermined signal level and supplies it as an intermediate frequency signal RIFs to frequency converters 17 and 18.
An oscillator 19 generates an oscillated signal VCR having a carrier wave frequency and supplies it to a phase shifter 20. The phase shifter 20 generates quadrature signals VI-CR and VQ-CR each having a phase difference of about 90° with respect to the oscillated signal VCR and supplies the signal VI-CR to the frequency converter 17 and the signal VQ-CR to the frequency converter 18.
The frequency converter 17 multiplies the intermediate frequency signal RIFs and the signal VI-CR to down-convert the intermediate frequency signal RIFs so that a baseband demodulated signal PI may be generated and supplied to a low-pass filter 21. Further, the frequency converter 18 multiplies the intermediate frequency RIFs and the signal VQ-CR to down-convert the intermediate frequency signal RIFs so that a baseband demodulated signal PQ may be generated and supplied to a low-pass filter 22.
The low-pass filter 21 extracts a signal having a desired baseband component from the demodulated signal PI and supplies it as a demodulated signal PIs to an A/D converter 23. Similarly, the low-pass filter 22 extracts a signal having a desired baseband component from the demodulated signal PQ and supplies it as a demodulated signal PQs to an A/D converter 24. It is to be noted that the low-pass filters 21 and 22 have a function of an anti-aliasing filter in order to prevent aliasing from occurring.
The A/D converter 23 converts the demodulated signal PIs into received digital data DI by sampling it with a clock signal CKa supplied from a clock signal generation portion 25 and supplies the data DI to a signal-processing portion 30. The A/D converter 24, on the other hand, converts the demodulated signal PQs into received digital data DQ by sampling it with the clock signal CKa supplied from the clock signal generation portion 25 and supplies the data DQ to the signal-processing portion 30. The signal-processing portion 30 performs error detection/correction processing etc. using the received data DI and DQ, to generate correct received data.
However, in the above-described receiving set, to obtain the demodulated signals PI and PQ from the intermediate frequency signal RIFs, it is necessary to have an oscillator for generating the oscillated signal VCR, a phase shifter for generating the signals VI-CR and VQ-CR each having a phase difference of about 90° with respect to the oscillated signal VCR, a frequency converter for multiplying each of the signals VI-CR and VQ-CR and the intermediate frequency signal RIFs, etc., so that the demodulated signal cannot be obtained by a simple circuit configuration.
Further, if, for example, a Gm-C type filter is used as the low-pass filters 21 and 22 for transformation into an integrated circuit (IC), 1/f noise has large influence because the low-pass filters 21 and 22 extract a baseband component, that is, a low-frequency signal. Further, since they do not have a large dynamic range, there may be such a case that it is necessary to have an attenuator for attenuating a signal to be input to the low-pass filters 21 and 22 or an amplifier for amplifying a signal output from the low-pass filters 21 and 22.
Furthermore, as the receiving set, such a direct conversion type receiving set is known as to directly generate the demodulated signals PI and PQ from the received signal RB by supplying the received signal RB to two frequency converters and further supplying one of these frequency converter with a signal VI-CR′ having a local-oscillated frequency close to a frequency of the received signal RB and the other converter with a signal VQ-CR′ having a similar local-oscillated frequency.
Such a direct conversion type receiving set needs to have two frequency converters for handling high frequencies, so that it requires large power dissipation and so is not preferable for a portable receiving set. Further, the frequency of the received signal RB and the local-oscillated frequency are close to each other, so that a signal of the local-oscillated frequency is liable to enter the received signal RB; to remove this influence, an order number of the filter or Q of characteristics of the filter must be designed to be high. Furthermore, if a high-order filter is used or a filter's Q is designed to high, the filter portion occupies a large chip area of an IC, if put in it, thereby increasing costs.